This invention is directed to a method and apparatus for treating fiber containing pulp, especially cellulose pulp, with an oxygen containing gas, in order to bleach the fiber containing pulp. The invention is expecially suited for treating cooked pulp directly passing from a continuous digestor. Gas suitable for the method according to the present invention may be either oxygen or an oxygen containing gas such as air.
Conventional methods are known wherein it is possible to remove lignin from cellulose pulp in order to obtain a greater degree of brightness of the pulp. Such conventional means involve treating the pulp by the use of oxygen gas or an oxygen containing gas in an alkaline medium employing increased temperature and pressure. It is also known that by use of certain protection substances, or so-called "protectors", it is possible to preserve the quality of the pulp which otherwise might be deturred due to the treatment. Prior to the present invention, installations for bleaching fiber pulp with an oxygen containing gas have required that the process be carried out in a container at an increased pressure, to which the fiber material, pulp, at a concentration generally from 20-30% in well shredded form has been fed into the top of a reactor, in which a temperature of 100.degree.-140.degree. C and a pressure of 1-12 atmospheres gauge has been maintained. Generally, the retention time of the pulp in such prior art reactors prior to dilution, mixing, and feeding from the reactor has been 30-60 minutes.
In a prior effort to improve the mixing and increase fiber surface area for reaction between the gas and fibers, a reactor has been designed with movable devices so that the pulp would fall stepwise from the top to the bottom of the reactor. In order to provide optimum contact in such a method between the pulp and the treatment gas, it has been necessary to maintain a relatively high concentration of the shredded pulp. In such methods, the shredded pulp forms a porous column containing the necessary quantity of reactive gas. However, when employing such high concentrations of the pulp, relatively little liquid is contained therein, and since the bleaching reaction process with oxygen, or oxygen containing gases, is exothermal, many difficulties have arisen when trying to control the reaction temperature with such known equipment. In an effort to overcome such problems, methods have been devised wherein certain quantities of the gas has been removed from the reactor, circulated through a cooling device and the cooled gas then re-introduced at a higher level into the reactor. However, it is difficult to achieve such a circulation in reactors having a large continuous pulp column wherein the gas is introduced therein.
Accordingly, it is the primary object of the present invention to provide a simplified procedure for bleaching cooked fiber pulp with oxygen containing gases.
It is also an object of the present invention to provide a suitable apparatus for bleaching cooked fiber pulp in a simplified manner.
It is a further object of the present invention to provide an apparatus arrangement which may be simultaneously utilized with the digestor pressure for direct feeding of the pulp into the reactor vessel, whereby commonly used apparatus may be eliminated from within the reactor.
It is a still further object of the present invention to provide a means and apparatus to recirculate reaction gas to the feed mixture thereby conserving the amount of gas necessary for said reaction.